1. Field of the Invention
This invention relates to a hydraulic pump unit used in a vehicle brake control system for an automotive vehicle that restrains an undesirable vehicle behavior, prevents a lock of wheels, generates a wheel-brake cylinder pressure of road wheels in response to a braking operation by a driver, or regulates the wheel-cylinder pressure of the wheels, in accordance with a current vehicle traveling condition.
2. Description of Related Art
It is known that there have been proposed various types of brake control systems for automotive vehicles that restrains an undesirable vehicle behavior, prevents a lock of wheels, or regulates a wheel-brake cylinder pressure of road wheels in response to a current vehicle traveling condition. These types of the brake control systems are disclosed in, for example, Japan Patent Publication (koukai) No. 10-6954 (corresponding to U.S. Pat. No. 5,964,510), the U.S. Pat. No. 6,234,588, the U.S. Pat. No. 5,893,896, and the PCT International Publication No. WO 98/13244, which designated the United States.
The Japan Publication No. 10-6954 discloses a pump unit associated with a hydraulic circuit adapted for a vehicle brake control system so called “traction control system” and “anti skid braking system”. The traction control system (hereinafter, referred to as “TCS”) restrains slippage of driving wheels that may happen when the driving wheels experience an excessive driving torque due to, for instance, acceleration and a sudden change of road surface conditions. The anti skid braking system (hereinafter, referred to as “ABS”) is provided to prevent a lock of the wheels that may occur when a braking force generated by a braking operation of a driver becomes greater than an appropriate braking force with respect to a road surface condition due to, for example, a sudden change of road surface conditions. The U.S. Pat. No. 6,234,588 discloses a brake control system so called “brake by wire system” that provides a braking force in response to the brake pedal operation of the driver. An amount of the brake pedal operation, such as a stroke of the brake pedal operation, depression pressure of the brake pedal operation, and pedal depression speed, is converted to an electric signal. The electrical signal is inputted into an electric control unit (hereinafter, refereed to as “ECU”). Based on the inputted signal, the ECU determines a current vehicle traveling condition, and creates an output signal to generate a reasonable braking force for the brake pedal operation. The U.S. Pat. No. 5,893,896 discloses a vehicular braking system that controls a stability of a vehicular attitude by regulating a braking force according to a variation in the vehicular attitude so as to stabilize the vehicular attitude. The PCT Publication No. WO 98/13244 discloses another brake control system so called “brake assist system” that controls a relationship between the brake pedal operation and a braking force. That is, there sometime is a demand for a stronger braking force than a braking force that a driver intended. The brake assist system provides an effective braking force considering vehicle traveling conditions in which a brake pedal operation is made, by detecting, for instance, a stroke of the brake pedal operation, depression pressure of the brake pedal operation, and pedal depression speed.
According to above described braking systems, there is provided a hydraulic pump unit that returns a surplus brake fluid to a master cylinder that is caused by a pressure-reduction procedure of wheel-brake cylinder pressure performed by such braking systems, or that provides brake fluid to wheel-brake cylinders in order to create the braking force, regardless of the brake pedal operation by the driver, or in response to the brake pedal operation. Such hydraulic pump unit is described in, for example, Japan Patent Publication (koukai) No. 7-47938. According to the publication, the hydraulic pump unit comprises a pump housing, a cylinder, an outlet valve, a plunger, an inlet valve, an inlet passage, an outlet passage, and a plug member. The pump housing has a hole and receives the cylinder therein. The cylinder includes the outlet valve at its bottom portion and receives therein the plunger with a slidable contact. Thus, a pump chamber is defined between a bottom surface of the cylinder and the plunger. The plunger has the inlet valve and is reciprocated by a cam. The inlet passage and the outlet passage have openings in the hole. An opening of the hole opposite to the bottom portion in an axial direction of the hole is closed by the plug member. By this arrangement, a pump function is performed. The hydraulic pump unit also has an orifice that reduces a pulsation generated by a reciprocative movement of the plunger. The orifice is provided in the outlet passage and comprises a main body and a hole. The main body of the orifice is assembled by fitting into the hole.
However, since the main body of the orifice is fitted into the hole, a part of components, i.e., metal shavings, might be shaved when the main body of the orifice is fitted into the hole.